Many products of commercial value need to be protected against counterfeiting, forging and copying. To this end, products of high value, such as perfumes or watches, as well as documents of value, such as banknotes, tax stamps, credit cards, membership cards, tickets etc., are typically provided with security markings. In order to improve the level of security and make the markings more difficult to counterfeit, typical security markings include for instance holograms, specific markings with luminescent dyes or pigments emitting in the visible spectrum upon excitation by e.g. UV radiation, watermarks, or graphical elements using a specific kind of pigment that is not easily available and/or which provides an optical impression by a specific orientation of the pigment that is difficult to achieve with commercially available equipment.
Security markings often employ luminescent materials, and the observed luminescence is used as authentication means. Herein, luminescence can be of two different types: fluorescence or phosphorescence. Fluorescence is the prompt emission of radiation upon excitation, whereas phosphorescence is the time-delayed emission of radiation, observable after the excitation has been stopped. Phosphorescence is characterized by a specific decay of the luminescence intensity in function of time; the corresponding lifetimes, which are material-specific, can range from the nanosecond to the multi-hour time scale. Neither type of luminescence can be obtained by using conventional black or colour toners as employed in e.g. photocopying machines, so that such luminescent markings cannot simply be copied.
Yet, still a drawback of such security markings is that they may be relatively easy to reproduce and/or may not be machine-readable. Further, while the optical impression of e.g. a luminescent marking as observed by the naked eye may be used as an authentication means at the place of control (e.g. at a place of sale, or ticket or entry control), many luminescent dyes are commercially available, and a counterfeiter may be able to mimic the optical appearance as observed by the naked eye. While a more sophisticated analysis of the spectral emission properties of a luminescent security marking may reveal that a marking that at first sight provides the same or similar optical appearance to the naked eye as a genuine marking is in fact forged or non-genuine, such a sophisticated analysis typically requires complex, expensive and bulky equipment that is typically not available at a place of control.
As one option for increasing the security level of a security marking on the analysis side, is it known to utilize and evaluate the decay properties of a luminescent (and in particular phosphorescent) marking. For instance, EP 1 158 459 A1 describes a method for authenticating a luminescent probe marking comprising the steps of exciting the luminescent probe marking with at least one excitation pulse, measuring probe intensity values of emission intensity (I) from emission radiation (E) of said luminescent probe marking in response to said at least one excitation pulse at time intervals, forming a probe intensity-versus-time emission function of said probe intensity values, and comparing said probe intensity-versus-time emission function with at least one reference intensity-versus-time emission function indicative for the genuineness of the marking, wherein said probe intensity-versus-time emission function and said reference intensity-versus-time emission function are normalized prior to comparison.
A drawback of this method is that pulsed excitation equipment is not easily available and may be bulky. Further, the observed intensity after a given time may be influenced by the surrounding light that may or may not excite and/or may or may not enter the detection device. Further, for relatively short decay times (i.e. fluorescence or phosphorescence with decay times of e.g. 10 ms or less), sophisticated equipment may be required to observe the intensity-versus-time emission function.
An early attempt to utilize the decay characteristics of a mixture of luminescent materials with different decay times is described in U.S. Pat. No. 3,412,245. In this document, it is described to mix two luminescent materials emitting at the same or very similar wavelength upon excitation. One of the materials has a significantly longer decay time than the other. Then, different (steady-state or DC and pulsed/alternating or AC) illumination conditions are employed, and a ratio of the signal observed under DC and AC illumination conditions is calculated and used as indicator for the presence of the respective luminescent components.
This method thus requires at least two different illumination conditions. Further, the analysis of the emission intensity and the emission profile may require complex equipment and is prone to disturbances if not performed in an isolated environment excluding for instance natural light.
A further approach relying on the decay time of a luminescent material as authenticating feature is described in WO 2005/095296 A2. Additionally, WO 2005/041180 A1 describes a photoluminescent marking where the emission intensity after a given time is compared with a reference value.
US 2013/0020504 A1 expands the above principle to security markings comprising two luminescent materials. These emit light at different wavelengths, and respective temporal intensity curves are determined. These are utilized to calculate initial intensities of the emission light components at a common time, and intensity parameters and decay parameter values for each of the emission light components are determined therefrom. The intensity parameters and the decay parameter values are then used for identification.
The document uses short light pulses of e.g. 500 μs, and hence requires suitable excitation light sources. The measurements can be repeated, and a large number of e.g. 256 measurements are used to increase accuracy. Also, rather sophisticated equipment is required for detection, as very precise intensity-time relations need to be established.
The document further emphasizes in paragraphs [0079] and [0080] that the decay constants are independent of the values of the maximal intensities, and that hence the decay constants can be used for authentication as such measured parameters allow deciding if a presence of the two distinct types of luminescent materials is confirmed or not. Conversely, the intensities as such cannot be used as means for authentication, as their magnitude depends not only on the material, but also on duration, intensity and wavelength(s) of the excitation light source as well as on the concentration of the photoluminescent material. This document thus focuses on the analysis of the behavior of the pure emitting species used in the security marking and aims at identifying the luminescent material. This identification is then used as authenticity criterion.
One drawback of the prior art methods is that typically complex, expensive and bulky equipment is required in order to provide for the desired illumination excitation conditions and/or to accurately determine the decay times of the materials. Further, prior art methods are limited to fluorescent or phosphorescent material having relatively short decay times of e.g. 10 ms or less, as a longer decay time will make a determination of the exact decay time constant more difficult and/or will prolong measurement for authentication.
Another drawback of the prior art methods is that the measurements are sensitive to the measurements conditions, and typically measurement in an isolated, specific apparatus or environment is required in order to avoid disturbances by e.g. natural light or fluctuations in illumination intensity, which may be caused by varying distances between illumination light source, security marking and detector, etc.
Further, the prior art typically relies on the luminescent properties of individual marker components (i.e. on the identity of the luminescent material), and hence only each component per se needs to be mimicked by a counterfeiter. This makes it difficult to modify a compromised (i.e. successfully forged) security marking such that the optical appearance to the naked eye is substantially maintained for the modified security marking as compared to the compromised marking, as then typically another material (i.e. another luminescent component) needs to be employed. Conversely, a relatively small change in the relative amounts of one or more luminescent components of the compromised marking will not be sufficient, as the decay characteristics of the marker remain unchanged.
The prior art methods thus typically do not allow modifying a compromised security marking such as to maintain its general appearance to the naked eye, while providing a different result in an equipment-assisted analysis. Further, for a given system of two or more luminescent materials, the prior art security markings and methods for their authentication generally do not allow obtaining clearly distinguishable markings and/or decisions on authenticity, as the methods are either insensitive to the composition (relative amounts) of the luminescent materials, or only depend on relative amount through absolute values of emitted intensities, necessitating strict control of excitation conditions and careful calibration of light sensors or through complex multi-exponential decay fitting operations for extracting several decay time constants of observed emission decay curve. The number of different authentication markers that can be realized by a system of two luminescent materials is thus typically very limited, e.g. one.
In a related aspect, the prior art methods also do not allow providing a link between security markings providing essentially the same general appearance to the naked eye, e.g. with regard to the observed colour, with another further item-specific, product-specific or batch-specific identification item, such as a batch or a range of serial numbers, a specific product line, a place of manufacture, a designated place of sale, etc., as different luminescent markers would need to be employed for the different security markings for the different batches, product lines etc., generally also leading to a different appearance to the naked eye.